We look at the possibility to compute and understand the color change occurring upon mutation of a photochromic protein. Accordingly, ab initio multiconfigurational quantum chemical methods are used to construct basic quantum-mechanics/molecular-mechanics (QM/MM) models for a small mutant library of the sensory rhodopsin of Anabaena (Nostoc) sp. PCC7120 cyanobacterium. Together with the wild-type forms, a set of 26 absorption maxima spanning a ca. 80 nm range is obtained. We show that these models can be used to capture the electrostatic change controlling the computed color variation and the change in the ionization of specific side chains.